The invention relates generally to production and purification of organic acids and more particularly to an improved processes for the production and purification of succinic acid from succinate salts that result from the fermentation of carbohydrates.
Succinic acid [110-15-6] (butanedioic acid), C.sub.4 H.sub.6 O.sub.4, is a naturally occurring constituent in plant and animal tissues, see, e.g., Winstrom, L. O. "Succinic Acid and Succinic Annydride", Kirk and Othmer Encyclopedia of Chemical Technology, Vol. 1, 4.sup.th Ed., (1978), the contents of which are incorporated herein by reference. It has therefore been affirmed as GRAS by the FDA. This status enables it to be used for various purposes, such as, but not limited to, a flavor enhancer, a pH control agent in foods such as condiments and for use in meat products. It is also widely used in scientific applications including uses in radiation dosiometry, standard buffer solutions, agriculture, foods, medicines, cosmetics, textiles, plating and waste-gas scrubbing.
Numerous patents discuss the production of carboxylic acids, such as succinic acid via fermentation. (See, e.g. U.S. Pat. No. 5,168,055 to Datta, the contents of which are incorporated herein by reference.) However, a major factor involved in industrial scale production using fermentation is the cost involved in downstream processing necessary to concentrate and purify the product. For example, it has been determined that fermentation proceeds best at an approximately neutral pH. However, the acid produced will eventually lower the pH. In order to avoid low pH fermentation broths that are injurious to the microorganisms driving the fermentation process, the pH of the broth should be raised by the addition of a base. However, the added basic material generally reacts with the acid and leads to the production of a salt of the carboxylic acid rather than the desired free acid product itself.
Thus, downstream processing typically involves both conversion of the salt into the free acid and the purification of the acidified product. Additionally, insoluble materials from the fermenter, such as the dead cells, generally need to be removed. Therefore, for fermentation to be economically viable, a technique for the production of relatively pure acid and an efficient recovery process is desirable.
U.S. Pat. No. 5,168,055 to Datta et al., the contents of which are incorporated herein by reference, proposed a process that combines the fermentation of carbohydrates to produce calcium succinate and the subsequent conversion to and purification of the succinic acid product. The succinate salt is acidified into the pure acid with sulfuric acid and gypsum, CaSO.sub.4, is produced as a by-product. The succinic acid produced is then processed through a series of steps designed to purify the product. However, it has been found that for every mole of succinic acid product produced, an equal amount of gypsum by-product is produced. This gypsum by-product has little value, in part, because the odor and color contamination from the fermentation process renders it unsuitable for commercial use. In addition, reagents such as calcium oxide or calcium hydroxide and sulfuric acid are consumed and are not regenerated within the process.
U.S. Pat. No. 5,143,834 to Glassner et al., the contents of which are incorporated herein by reference, proposes a similar combination of fermentation and purification processes for the production of succinic acid from disodium succinate that is formed in the fermentation step. Succinic acid is produced by using a combination of electrodialysis and water splitting steps that ultimately separate the base, and produce pure acid. Further purification is achieved by passing the product stream through a series of ion-exchange columns. However, this process has disadvantageously high costs, such as membrane costs and the electrical energy costs associated with electrodialysis.
U.S. Pat. No. 5,034,105 to Berglund et al., the contents of which are incorporated by reference, proposes a process for obtaining a carboxylic acid of high purity by using water splitting electrodialysis to convert an undersaturated aqueous solution of disodium succinate into a supersaturated solution of succinic acid that facilitates in crystallizing the product carboxylic acid. However, this process also suffers from the high costs associated with the Glassner et al. patent.
Accordingly, it is the objective of this invention to provide an improved method of producing and purifying carboxylic acids, such as succinic acid, which result from fermentation processes.